Fuel evaporative emission control systems have been in use on automotive vehicles for over 30 years. The gasoline fuel used in many internal combustion engines is quite volatile. The fuel typically consists of a hydrocarbon mixture ranging from high volatility butane (C-4) to lower volatility C-8 to C-10 hydrocarbons. When a vehicle is parked in a warm environment during the daytime heating (i.e., diurnal heating), the temperature in the fuel tank increases. The return of hot fuel from the engine also heats the contents of the fuel tank. The vapor pressure of the heated gasoline increases and fuel vapor will flow from any opening in the fuel tank. Normally, to prevent fuel vapor loss into the atmosphere, the tank is vented through a conduit to a canister containing suitable fuel adsorbent material. High surface area activated carbon granules are widely used to temporarily adsorb the fuel vapor.
The fuel vapor enters the canister through a top inlet of the canister and diffuses downwardly under its own pressure and gravity into the bed of carbon granules where it is adsorbed in temporary storage. The total volume of adsorbent is specified so as to be suitable to retain a quantity of fuel vapor expected to evaporate from the fuel tank during normal or representative usage of the vehicle.
The canister is molded of a thermoplastic material and shaped so that ambient air can be drawn through the carbon granule bed during engine operation to purge adsorbed fuel from the surfaces of the carbon particles and carry the removed fuel vapor into the air induction system of the vehicle. Typically, a partition is formed in the canister to lengthen the flow path of vapor and air through the volume of carbon particles. Thus, the fuel vapor enters at one end of the flow path and escapes to the atmosphere at the opposite end, the vent end, if the quantity of fuel exceeds the adsorption capacity of the carbon volume. Ambient air, induced to flow through the activated carbon bed under engine intake vacuum, enters the canister at the "vent" end of the flow path. The air traverses the full length of the flow path and exits the canister with desorbed, i.e., purged fuel at the vapor inlet end of the carbon volume. Typically, neither the canister nor the purge air experience heating other than ambient heating.
The described emission control system obviously works in a repeating cyclical mode. When the engine is not running, fuel vapor generated by diurnal heating, or the like, flows to the canister and is adsorbed up to the capacity of the adsorbent volume. The vehicle may remain idle for several days and fuel vapor will accumulate in the canister. The initial loading will be at the inlet end of the adsorbent volume but the fuel gradually becomes distributed along the entire adsorbent bed pathway. When the vehicle engine is started and can accommodate a secondary fuel-air mixture, a purge valve is opened and purge air is drawn through the adsorbent volume. Purging can continue as long as the engine is running and the air can cause the removal of a substantial portion of the stored fuel vapor. But a portion of the adsorbed hydrocarbons remain adsorbed on the carbon. That portion is called the "heel" and it significantly limits the capacity of the carbon to adsorb additional fuel.
Environmental regulators are proposing lower limits on the amount of fuel vapor that can escape the evaporative emission system during a prescribed test of the system in a closed space called SHED (Sealed Housing for Evaporative Determination). For example, the California Air Resources Board (CARB) has proposed "near zero" and "zero" evaporative emission standards for automotive vehicles for year 2004. The proposed standards require near-zero fuel vapor emissions from all the sources: permeation losses through plastic fuel system parts; leaks through the fittings and joints; and canister breakthrough emissions. Reducing the emissions through the leaks involves the selection of better sealing joints and connectors or eliminating some of joints, and reducing permeation losses involves the selection of low permeability or no permeability materials, whereas reducing canister breakthrough emissions to near-zero requires new technologies in the canister design. An object of this invention is to provide a canister system, and method of operating the system, that will limit canister breakthrough emissions to less than 0.02 grams fuel loss per test.